The proposed research aims to develop the mouse mammary tumor system as an experimental model for cancer immunotherapy, to reveal fundamental aspects of antitumor immunity, and to indicate effective and hazardous immunotherapeutic procedures. Areas of investigation are primarily four: 1. To determine the antigenic strength, frequency of occurence and interrelationship of virus-associated antigens and tumor- specific antigens in mammary tumor virus (MTV) infected and in MTV-free mice. The assay system will use standard sensitization-challenge methods. 2. To determine the effect on host resistance of specific immunotherapy in syngeneic and in autochthonous tumor hosts using killed tumor cell preparations and antigen-retaining RES organ preparations. The tests will compare the effects of route and vehicle of administration of different immunizing preparations, and the effects of timing and amount of preparation administered to hosts at different levels of specific resistance. 3. To study the variables in the immunization procedures which are critical in the stimulation of immune resistance factors while minimizing the risk of antibody-mediated enhancement of tumor growth. The development of cellular and humoral immune factors will be assayed by passive transfer to normal or irradiated (400R) unsensitized recipients at the time when the recipients are challenged with implants of living tumor cells. 4. The development of in vitro methods to assess both the humoral and cell mediated immune status of tumor bearing hosts. The Colony Inhibition test and the Migration Inhibitory Factor test will be adapted to test cells and sera from mice carrying spontaneous tumors at various stages of development and at different stages of immunotherapeutic procedures.